Timing recovery for channels with binary modulation

Abstract

For (re)writable and read-only optical disc systems the data clock is recovered by a phase locked loop (PLL), where the error signal is generated by comparing the actual zero-crossing with the zero-crossing of the generated clock signal. Given an optical system with a laser wavelength λ laser and a numerical aperture NA, the cut-off wavelength of the Modulation Transfer Function is given by λ 0 =λ laser /(2·NA). With decreasing bit length the amplitude of the minimum wavelength will decrease and is zero for wavelengths below λ 0 . Consequently, the phase error signals generated by zero-crossing of these signals are disturbed by noise. The idea of the invention is to use the zero-crossings with sufficient performance only in deriving phase information for the clock recovery.

1 . A method of providing reliable phase error signals in a phase locked loop ( 20 ) in an optical system, which optical system is adapted to read data from an optical disc, said method comprising the steps of: 
 reading a bit pattern on the optical disc, thereby providing a plurality of signal samples (Sk),    feeding the signal samples (Sk) to a first phase detector ( 40 ,  41 ,  42 ,  43 ) in the phase locked loop ( 20 ),    using a changing of polarity of successive signal samples (Sk), a so-called zero crossing, in the first phase detector to generate a phase error signal for the phase locked loop ( 20 ), characterized in that    the first phase detector ( 40 ,  41 ,  42 ,  43 ) is adapted to take into account the polarity of a number of signal samples (Sk) before and after a zero crossing to derive a reliable phase error signal so that the influence of noise and Inter Symbol Interference (ISI) is reduced.    
     
     
         2 . A method according to  claim 1 , characterized in that the number of signal samples (Sk) used before and after a zero crossing depends on the quality of the signal samples (Sk).  
     
     
         3 . A method according to  claim 1 , characterized in that the data on the optical disc are stored in Binary Modulation (BM).  
     
     
         4 . A method according to  claim 1 , characterized in that the first phase detector ( 40 ,  41 ,  42 ,  43 ) is adapted to take into account only those changes of the polarity which are preceded by at least n signal samples with identical polarity and which are succeeded by at least n signal samples with the opposite polarity.  
     
     
         5 . A method according to  claim 1 , characterized in that the data on the optical disc are stored in Run Length Limited (RLL (d)) encoding with a constraint d being the run length.  
     
     
         6 . A method according to  claim 5 , characterized in that the first phase detector ( 40 ,  41 ,  42 ,  43 ) is adapted to take into account only those changes of the polarity which are preceded by at least n signal samples with identical polarity and which are succeeded by at least n signal samples with the opposite polarity, where n satisfies the condition: n≧d+2.  
     
     
         7 . A method according to  claim 6 , characterized in that the data on the optical disc are stored in RLL (d=1 encoding.  
     
     
         8 . A method according to  claim 4 , characterized in that the method further comprises: 
 feeding the signal from the first phase detector ( 43 ) to an Anti-Aliasing Filter ( 44 ) and subsequently to a second phase detector ( 45 ).    
     
     
         9 . A method according to  claim 8 , characterized in that, the first phase detector ( 43 ) takes signal samples, where the minimum run length is 2 into account only, and the second phase detector ( 44 ) takes signal samples, where the run length is equal to or greater than 3 into account only.  
     
     
         10 . A system for performing the method according to  claim 1 .  
     
     
         11 . An apparatus for writing bit patterns on an optical disc to be read in by use of method according to  claim 1 .  
     
     
         12 . A disc whereon bit patterns are written to be read by use of the method according to  claim 1.